Various apparatus and methods have been devised to reduce sewer odors that are released from manholes. These include sealed covers, inserts with activated carbon and odor control materials, chlorine and chemical treatment and mechanical ventilation systems. Most of these methods do not deal with the fluctuations of sewer gas and ambient air movement in and out through the manhole cover. Frequently it is not practical or economic to provide some of these prior art methods such as chemical treatment or forced ventilation. Many sewer systems have just a few isolated manholes that have odor problems where an odor absorbing type insert can quickly be used to reduce odor complaints from pedestrians and businesses.
Sewer manholes customarily are disposed within a structure that has a relatively large volume of sewer gas above the sewer slurry and liquid in fluid communication with the flow portion of the system piping and the associated manhole covers. A slight change in sewer flow rate, chemical activity, temperature, ambient air pressure or wind velocity can cause an emission of gases from manhole covers or an inflow of ambient air into the sewer system. The gas pressure and the volume of gases within the system is not constant. Numerous environmental, biological, chemical, sewer flow rates and other conditions cause the gases within the system to be formed or displaced, expanded or contracted along with outside influences such as wind velocity over the grate, outside temperature and influences such as the fluid communication with other manholes, pumps, and flow streams. The lower the fluid in the sewer pipe the greater the volume above the slurry in the pipe. This is where many odorous gases are formed. Some municipalities have complained that the low flow plumbing shower and toilet fixtures have added to the odor problems by reducing flow rates that result in less scouring of the pipe interiors.
No two manholes are exactly the same as to emissions of sewer gases. From normal well known odor complaints by the public and experience, what is well known is that in certain manhole areas, they have very objectionable odors that occur at random times and at varying intensity. The sewer gas emissions and air egression into a manhole vary in volume size from very small to large quantities. The frequency of these fluctuations, also vary widely but certain times and conditions are more predictable problem periods. For example during time periods where more people are using plumbing fixtures at the same time over low use night time periods.